This project is designed to study the role that pancreatic insulin deficiency plays in some of the health problems affecting children with cystic fibrosis. The specific aims of the study are: (1) to evaluate the effects of insulin deficiency on growth and health in cystic fibrosis and (2) to determine if correction of the insulin deficiency by administering tolbutamide (an oral hypoglycemic agent) improves the ability of the pancreas to secrete insulin.